In response to the increasing requirements of the Internet, multimedia and the like, computer technology has great advances. The required hardware structures such as hard disk drives, optical disk drives, burners have become basic equipment of users or server operators. The required output power provided by the power supply device also has to increase at the same time. The basic power of 250 W in the past no longer meets the requirements. These days the computer hardware equipment often requires 450 W, 500 W or 600 W of output power.
However, the prevailing trend of hardware design at present is to maintain the function of the large equipment at a shrinking size. The power supply also has to follow this trend. This causes problems in the design of power supply devices. For instance, for the power supply device used in the common 1U specification, the maximum width is 100 mm, and the electric connection board for AC and DC output and input has a width of 92 mm. Due to the present technique is to wire the AC input and DC output on a single circuit board on which AC input legs occupy 22 mm. The remained 70 mm is for DC output. However, convert the power to output legs, every 100 W of power needs 15 mm. With the present 1U specification, the maximum power supply output is 450 W. It becomes a capacity constraint. As the present server space requires at least 500 W of power supply, or at least 75 mm for the DC output legs. The present technique cannot meet this requirement. To change the width or space design of the server is difficult. Hence this is an important technical issue pending to be resolved in the power supply industry.